This application claims priority from Japanese Patent Application No. 2002-317865, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a support structure for a blanket cylinder of an offset printing press designed particularly taking into account the relationship between throw-on and throw-off operations of the blanket cylinder and printing pressure applied to an impression cylinder of the blanket cylinder.
2. Related Art
In the offset printing press, a blanket cylinder, also called as a rubber cylinder, is provided between a plate cylinder and an impression cylinder. This blanket cylinder has an axis movable so as to enable the blanket cylinder to be xe2x80x9cthrown onxe2x80x9d, or drawn into engaging position, relative to the plate cylinder and the impression cylinder at the start of the printing operation, and xe2x80x9cthrown outxe2x80x9d, or drawn out of the engaging position, relative to both cylinders at the end or interruption of the printing operation. In addition to the throw-on and throw-off operations, a printing pressure adjustment is performed every time the thickness of printing sheets such as sheets of paper (hereinafter referred to only xe2x80x9csheetsxe2x80x9d) are to be changed in order to provide a printing pressure suitable for those sheets having a different thickness by adjusting the clearance between the blanket cylinder and the impression cylinder according to the thickness of the sheets, when the blanket cylinder has taken the throw-on position relative to the impression cylinder.
As a commonly applied arrangement in order to achieve the above throw-on/throw-out operations and the printing pressure adjustment as described such as in Japanese Unexamined Patent Publication (Kokai) Nos. HEI-04-226354, 2001-113669 and HEI-10-315429, the blanket cylinder is rotatably supported by double, outer and inner eccentric bearings mounted in the frame of the printing press. Specifically, the frame rotatably supports the outer eccentric bearing that in turn rotatably supports the inner eccentric bearing that in turn rotatably supports the blanket cylinder. Accordingly, by the rotation of the outer eccentric bearing, the blanket cylinder is moved via the inner eccentric bearing, thereby achieving the printing pressure adjustment. On the other hand, by the rotation of the inner eccentric bearing with the outer eccentric bearing locked in position, the blanket cylinder is moved, thereby achieving the throw-on/throw-out operations.
In the above support structure for the blanket cylinder, the adjustment of the clearance between the impression cylinder and the blanket cylinder on the basis of the thickness of printing sheets causes change in distance between the axes of the plate cylinder and the blanket cylinder (a center-to-center distance). If this change falls within an acceptable range, excessive load is unlikely to be applied to the plate cylinder or the blanket cylinder and therefore a printing trouble is not caused.
However, where the distance between the plate cylinder and the blanket cylinder becomes excessively smaller than an acceptable value due to an excessively large adjustment of the printing pressure such as for sheets having a thickness larger than a predetermined value, an excessive load is applied to the plate cylinder and the blanket cylinder, which may result in printing trouble. In order to avoid this trouble due to the small distance out of the acceptable range, an adjustment mechanism for moving the axis of the plate cylinder may be provided as proposed such as in Japanese Unexamined Patent Publication (Kokai) No. HEI-10-315429, but such mechanism complicates the structure of the printing press and hence increases the manufacturing cost.
The present invention has been conceived to address the above problems. It is an object of the present invention to provide a support structure for the blanket cylinder of the printing press that is capable of reducing the possibility to cause a printing trouble even in the printing operation for thicker sheets without the necessity to provide the adjustment mechanism enabling the movement of the axis of the plate cylinder.
According to the present invention, there is provided a support structure for a blanket cylinder of an offset printing press, which includes: a first eccentric bearing that is rotatably supported by frames of the offset printing press; a second eccentric bearing that is rotatably supported by the first eccentric bearing so as to rotatably support an shaft of a blanket cylinder; a printing pressure adjustment device that rotates the first eccentric bearing relative to the frames, thereby adjusting a clearance between an impression cylinder and a blanket cylinder; and a throw-on device that rotates the second eccentric bearing relative to the first eccentric bearing, thereby performing throw-on and throw-out operations. In the thus arranged support structure, a throw-on distance of the throw-on device is variable.
With the thus arranged support structure, the throw-on distance, that is, the change amount of the clearance between the blanket cylinder and the impression cylinder in the throw-on operation is variable so that, for example, when sheets are changed from thinner ones to thicker ones, the throw-on distance is set to be smaller, thereby enabling the printing pressure adjustment device to have a decreased adjusting amount corresponding to the decreased amount of the throw-on distance.
Preferably, the throw-on distance of the throw-on device is switchable between a standard throw-on distance and a thick-sheet throw-on distance (i.e., a throw-on distance for thick sheets). The thick-sheet throw-on distance is smaller than the standard throw-on distance. In the switching between such two alternatives, a switching mechanism in the throw-on device can be simplified so that the switching action can be securely and easily made whether or not it is manually or automatically made.
The support structure preferably further includes a control device that controls the printing pressure adjustment device and the throw-on device on the basis of the thickness of sheets to be printed. The control device switches the throw-on distance of the throw-on device from the standard throw-on distance to the thick-sheet throw-on distance and designates the result determined by subtracting the difference between the standard throw-on distance and the thick-sheet throw-on distance from the change amount of the thickness of sheets as the adjusting amount of the printing pressure adjustment device where the thickness of sheets before changing is less than a predetermined value and the thickness of sheets after changing is equal to or more than the predetermined value. Further, the control device switches the throw-on distance of the throw-on device from the thick-sheet throw-on distance to the standard throw-on distance and designates the result determined by subtracting the difference between the standard throw-on distance and the thick-sheet throw-on distance from the change amount of the thickness of sheets as the adjusting amount where the thickness of sheets before changing is equal to or more than the predetermined value and the thickness of sheets after changing is less than the predetermined value. Further, the control device does not switch the throw-on distance of the throw-on device and designates the change amount of the thickness of sheets as the adjusting amount of the printing pressure adjustment device where both the thicknesses of sheets before and after changing are either less than, or equal to or more than the predetermined value.
With the above arrangement, the switching operation is also automatized. As a result, the work load can be greatly reduced.
In the case where the support structure is designed for supporting plural blanket cylinders of a multi-color printing press, the printing pressure adjustment device and the throw-on device are provided for each of the plural blanket cylinders. In this arrangement, the control device makes the printing press incapable of printing where the throw-on distances of all the throw-on devices are not the same as each other. Thus, the control device can prevent occurrence of broke even if the throw-on distances of the respective blanket cylinders are not the same as each other.